1. Technical Field
The present invention relates to a fixing device for fixing an image on a recording medium used in an image forming apparatus, and more particularly, to a heat roller mechanism for locking movement of a heat roller when the heat roller is rotated by a driving gear to fix a toner image on the recording medium.
2. Background Art
In an image forming apparatus such as, for example, a copier, a printer, an a facsimile machine etc., using an electrophotographic process, a latent image formed on a photosensitive drum is developed by applying toner from a developing unit onto the photosensitive drum. The toner image is then transferred and fixed on a recording medium. When a toner image is fixed on a recording medium, the toner image is first heated and fused onto the recording medium, and then naturally cooled so that it is fixed onto the recording medium.
In a conventional fixing device used for fixing an image on a recording medium as described, for example, in U.S. Pat. No. 5,485,260 for Fixing Device, Fixing Method, And Recording Apparatus issued to Mitsuya, U.S. Pat. No. 5,293,202 for Image Fixing Apparatus issued to Adachi et al., and U.S. Pat. No. 5,221,947 for Internally Heated Roller Assembly For Toner Image Fixing Apparatus issued to Ndebi et al., a pair of coactive fixing rollers consisted of one heat roller and one support roller is extensively used. The heat roller is typically heated to bring the pair of coactive rollers into contact with each other, thereby forming a nipping and fusing section. The recording medium is passed through this nipping and fusing section to fix the toner aligned thereon. When the recording medium is passed through the nipping and fusing section, the aligned toner which forms an image on the recording medium is heated and at the same time subjected to pressure. The heat energy and pressure applied at the nipping and fusing section changes the shape of the toner. This action causes the toner to be fixed onto the recording medium.
The heat roller is usually a hollow cylinder made of aluminum and has a heater at its central section. The aluminum hollow cylinder is called a core metal. A halogen lamp is often used for the heater. The support roller, on the other hand, has an elastic layer formed along the outer periphery of its metallic rotating body as described, for example, in U.S. Pat. No. 4,942,434 for Fixed Roller For An Electrostatic Image Recorder issued to Nakai et al. When it is pressed in contact with the heat roller, the elastic layer changes shape to form a nipping and fusing section. The heat roller of the conventional fixing device is typically supported by opposite end cylindrical bearings including a projection attached to an internal base of the frame of the fixing device. Conventional bearings are configured to prevent abrasion to the heat roller. A typical example of such conventional bearings is shown in Adachi et al. '202. Another bearing configuration is described in U.S. Pat. No. 5,287,156 for Fixing Device Capable Of Separating An End Portion Of A Recording Material By A Bearing issued to Shikada et al., in which fixing roller bearings are constructed, in lieu of a separation guide, to separate a recording medium from the surface of the heat roller in order to eliminate separation failure and damage to the surface of the heat roller. In addition to the opposite end cylindrical bearings as shown for example in Adachi et al. '202, stoppers in a form of a C-ring, as I have noted, are necessarily required to limit movement of the heat roller in its longitudinal direction in order to ensure that a toner image is cleanly fixed on a recording medium. The requirement of the pair of stoppers in addition to the bearings is however unproductive. I have also observed that the installation of such stoppers in the fixing roller assembly requires not only additional production cost but also consumes valuable assembly time both of which are highly undesirable in a production environment.